One property of a fault that might be influenced by maturity is its strength. The strength of a fault determines how long that fault can resist increasing shear stress before it slips, causing an earthquake (or aseismic slip, in the case of a very weak fault). Shear stress is stress oriented parallel to the slip direction of the fault. Fault rupture occurs when the shear stress on a fault overcomes that fault's strength, a value determined by several different factors. This can be expressed mathematically as follows:
Cohesion is a term that describes how well the two sides of a fault "cling" to each other -- a resistance to movement independent of friction. Friction is a force that acts against the sliding of the fault surfaces past each other. Normal stress is stress oriented perpendicular to the plane of the fault, and pore pressure is the pressure that fluids within a rock exert on their surroundings.
How might these values change as a fault matures? The more mature a fault is, the more its two sides have slipped past each other. The fractured rocks in the Earth's crust are similar to any other rough material -- the more you grind them against each other, the smoother they generally become. As rough surfaces become smoother, the friction between them tends to go down, as does their tendency to "interlock" (their cohesion). In addition, the ground-up rock (like sawdust from cutting or sanding wood) remains within the fault, and can act as a kind of lubricant, making each successive fault slippage easier. Geologists refer to this finely ground-up rock as fault gouge.
Given this simple reasoning, it seems sensible to expect that mature faults might have lower values for friction and cohesion than would immature faults. Both of these changes, according to our formula above, would lower the strength of a fault, all else being equal.
